A well-known technique for separating gasses is pressure swing adsorption (PSA). An important application of the PSA process is in PSA dryers which reduce the concentration of various gasses or impurities in a compressed air stream. For example, PSA dryers are used to remove water vapor from air entering nitrogen and oxygen liquefiers. Nitrogen and oxygen liquefiers comprise two integrated distillation columns which operate at very low temperatures. Consequently, it is essential that water vapor and carbon dioxide be removed from the gas flowing into the liquefier. Without such pre-purification, these impurities will condense in the low temperature sections of the liquefier, blocking the flow. In order to prevent freeze up, the content of water vapor and carbon dioxide in the gas stream entering the liquefier should be less than a few parts per million (μg/g).
A conventional PSA dryer 4 is illustrated in FIG. 1. The device operation involves alternating forward and backward cycles. In the forward cycle illustrated in the figure, a stream of compressed air 10 downstream from a compressor 6 enters dryer 4 and is directed through a four-way valve 12 to pass through a first column 14 containing a molecular sieve 28 or other adsorbent which adsorbs water vapor present in the stream. A product flow 11, consisting of most of the dry air exiting the column 14, flows through a first one-way check valve 20 and exits the device 4 as a purified product stream 22. A purge flow 13, containing a residual portion of the dry air exiting the column 14, passes through a restrictive aperture or metering capillary 18 where its pressure drops and its volume increases. The dry purge flow 13 then passes through a second column 26 containing an adsorbent 30. The dry flow desorbs water vapor present in the column 26, and the resulting moist flow is directed through four-way valve 12 so it is vented from device 4 as a moist exhaust flow 16. After a period of time, the four-way valve 12 is switched to reverse the direction of flow from forwards to backwards, i.e., so that the flow is circulating clockwise rather than counter-clockwise. The operation of the device during the backward cycle is analogous to the operation in the forward cycle. After operating for a period of time in this backward cycle, the four-way valve 12 is again switched, causing the flow to reverse again.
During each cycle, one of the two columns 14 and 26 is adsorbing moisture while the other column is being purged of moisture that it had adsorbed in the previous cycle. Even though the purge flow represents only a minority of the high pressure flow by weight, the volume of the low-pressure purge flow taking up moisture from one column is actually larger than the volume of the high-pressure input flow giving up moisture to the other column. Consequently, the dry purge flow is able to remove nearly all the moisture adsorbed by the column in the previous cycle, preparing for the next cycle when the column will again adsorb moisture from the entering high-pressure flow. Using commonly available molecular sieve adsorbents, the PSA dryer can reduce the water content of the incoming stream 10 so that the concentration of water in product stream 22 is a few μg/g or less.
In many applications where PSA dryers are used, however, it is also important to reduce the concentrations of carbon dioxide, sulfur dioxide, oil vapor, and other secondary substances to a few μg/g. For example, nitrogen and oxygen liquefiers are quickly clogged by condensation of carbon dioxide and other impurities if the concentrations of these secondary substances are larger than a few μg/g.
One approach to reducing the concentration of these secondary substances is to provide an irreversible filter external to the PSA dryer. As shown in FIG. 1, a pre-filter 8 may be positioned between the compressor 6 and dryer 4 to remove a secondary substance from the compressed air prior to entering the PSA dryer. Alternatively, the filter 8 could be placed downstream from the dryer 4 to filter product flow 22. As an example, filter 8 may be a small irreversible filter made of asbestos coated with sodium hydroxide. Such a filter can be used to adsorb carbon dioxide. These filters are commonly used in night-vision Joule-Thompson cryogenic coolers. Periodically, however, the filter becomes saturated and must be disposed and replaced. In addition to adding expense to the device, these filters generate toxic waste and are dangerous to handle. Thus, it would be desirable to eliminate such irreversible filters, if possible.
Even without the irreversible filter 8, the PSA dryer 4 will itself reduce the concentration of some secondary substances, but only to a limited degree. For example, using molecular sieve adsorbents 28 and 30 that are able to adsorb molecules as large as 1 nanometer in diameter, the PSA dryer 4 can reduce the concentration of carbon dioxide, sulfur dioxide, oil vapor, and other secondary substances. However, these adsorbents conventionally used in PSA dryers have a significantly higher affinity for water vapor than for these secondary substances. Consequently, a PSA dryer that reduces the water content to a few μg/g will not reduce the concentration of secondary substances to equally low values. Thus, the product stream 22, while having reduced concentrations of secondary substances such as carbon dioxide, these concentrations remain significantly larger than a few μg/g.
One way to modify the PSA dryer to further reduce the concentration of secondary substances in the product stream 22 is to increase the size of the adsorbent columns 14 and 26 and volume of adsorbents 28 and 30. This solution reduces the concentrations of both water and secondary substances in the produce stream 22. Because the larger adsorbents significantly increase the size, weight and cost of the PSA dryer, this approach is practical only for large-scale industrial PSA devices. Another approach is to increase the amount of the purge flow 16, which demands a significantly larger compressor 6 to provide higher pressure. Consequently, this solution also increases the size, weight, and cost of the device, as well as increasing its power consumption. This solution, therefore, is useful only in large-scale industrial applications, and is not practical in small, compact PSA dryers.
Another approach to reducing the concentrations of secondary substances in the product flow 22 is to provide a more effective adsorbent 28, 30 in the PSA columns 14, 26. For example, U.S. Pat. No. 6,638,340 to Kanazirev, et al. discloses a solid adsorbent material composed of a zeolite, alumina, and metal. The adsorbent is designed to remove multiple contaminants such as water and carbon dioxide from an air stream during a PSA process. This adsorbent, however, is not readily available and requires careful handling because of the reactive metals used in the preparation. Consequently, it is relatively costly to prepare. Moreover, a PSA dryer using this adsorbent needs a large purge flow and thus a large compressor. This adsorbent, therefore, is only suitable for use in large PSA dryers in an industrial plant. Similarly, U.S. Pat. No. 6,358,302 to Deng, et al. discloses a multi-composite adsorbent that comprises one water vapor removal adsorbent, one carbon dioxide removal adsorbent, and a third adsorbent which can selectively adsorb hydrocarbons and/or nitrogen oxides. The carbon dioxide adsorbent is a zeolite, and the water adsorbent is an activated alumina, silica gel, or non-zeolite desiccants. This adsorbent, however, has a weaker affinity for carbon dioxide than for water. Thus, a PSA dryer using this adsorbent has the disadvantage that it needs a large purge flow, and hence a large compressor. In view of these problems with the current state of the art, it would be an advance in the art of PSA dryers to overcome these various disadvantages that are especially relevant to the need for compact PSA dryers used in smaller scale applications.